This invention is directed to injection molding machines, and more particularly, to an injection molding machine having open access to the mold area on at least three sides and which includes a mechanism for correcting misalignment of mold platens caused from deformation of the molding machine frame during clamp-up.
Injection molding machines without tie bars on the top of the mold platen are preferable for they allow ease of access to the mold area. However, these machines typically cause clamping forces to be absorbed by the frame of the injection molding machine. The advantages of a machine not having tie bars is that the mold area defined by the mold halves remains substantially unobstructed and can be easily accessed for allowing easy mold installation, removal, or servicing. Accordingly, larger molds can be accommodated; more flexibility in implementing product handling devices is acquired; and easier part removal is achieved. The major flaw with injection molding machines having tie barless tops is that because the frame of the machine absorbs clamping forces, the frame is subject to deformation which leads to misalignment of the mold platens.
The several machines which follow have been designed for addressing the problem of frame deformation for top tie barless machines, however, none have been entirely successful.
European Patent Specification 0311133 represents one device employing articulation between the movable mold platen and the closing piston. The articulation compensates for any deformation in the machine frame but significantly increases the cost of the device. Additionally, such devices are particularly prone to fretting corrosion due to small movements and high loads in the articulated joint.
U.S. Pat. No. 4,222,731 to Enrietti et al. discloses a device having a longitudinal member pivotally connected to the central portion of two other members, wherein one end of each other member is connected to one of the platens. The opposite ends of the two other members are connected by a clamping cylinder piston. One of the two other members is divided into two portions, wherein the two portions have a releasably rigid connection therebetween. A separate piston cylinder is mounted to the machine frame for causing the moving platen to move between a mold open and a mold closed position. During the mold opening and closing strokes, the ends of the divided lever 22 swing back and forth past the member 20. Since the levers 22 are designed to withstand full mold clamping force, they must be sized accordingly, such that they are generally very large. Therefore, energy is wasted in moving the large mass of the levers 22 during each molding cycle.
Enrietti et al. also employs sliding connections between channel-shaped guides 24 of moving platen 6 to compensate for deformation. However, such sliding and/or pivoting connections are prone to wear and by definition, exert a high load over a small area resulting in concentrated loads. The stationary platen 2 is connected to one of the two members 16 and injection unit 10 moves back and forth once during each cycle with moving platen 6. This arrangement needlessly complicates the apparatus and is also a waste of energy, slowing the operating speed of the machine.
U.S. Pat. Nos. 5,249,951, 5,297,952 and 5,332,385 disclose several additional embodiments of a tie barless machine. All of the apparatuses disclosed incorporate means for resisting the clamping forces via the frame of the machine, exclusively. In each case, the machine frame deforms during the clamping step and therefore each machine employs one or more tilting platens to compensate for misalignment of the platens resulting from the deformation of the machine frame when the clamping force is applied.
U.S. Pat. No. 4,389,183 discloses an injection molding machine wherein the clamping forces are resisted by two hollow rectangular frames connected only by the stationary platen and a bed on the other end. There are no tie bars in this device but the rectangular frame still obstructs access to the area between the molds.
U.S. Pat. No. 4,645,443 discloses a means for adjusting shut height on an injection molding machine using a threaded nut supported on the back of the moving platen. When the nut is rotated it screws on the end of the clamping piston either towards or away from the moving platen. Since this adjustment mechanism is attached to the moving platen, it must shuttle back and forth with each machine cycle. This shuttling step requires additional energy and complexity to operate the device. Since the shut height is only adjusted when a machine is being set up to run a particular mold, this arrangement is a needless complication.
Accordingly, each of the devices discussed above include limitations to which the instant invention is directed to overcome. These limitations include deformation of the machine frame which results in misalignment of mold platens; complex compensation devices to correct the parallelism of the platens, requiring increased costs and additional space; substantial mass used to form the frame in order to minimize the deformation of the frame, increasing the costs and space consumption of the machine; wasted energy and decreased speed of machine operation due to the heavy weight of deformation resistant elements.
There exists a need, therefore, for an injection molding machine which allows for easy access to the mold area and which efficiently compensates for frame deformation during the clamp down process.